Carbon dioxide gas is widely-known to have an extremely large effect on expansion of the blood vessels. Within a carbon dioxide range of 25 through 600 mmHg, the change in blood flow rate in the brain is nearly linear in response to carbon dioxide levels in the arterial blood (non-patent document 1, Yamashita and others, “Brain and circulatory blood pressure are adjusted by the brain” p. 144–145, Kyoritsu Publishing Company, (1998)). The interrelation of carbon dioxide concentration within the blood, and the blood flow rate has been applied to control the concentration of carbon dioxide during respiration (patent document 1, JP-A No. 224089/2002). This interrelation has been applied to measurement with PET for early stage diagnosis of apoplexy. PET (or positron emission tomography) is a technology for non-invasive measurement and imaging of blood and blood flow within a living body. In this method, radioactive isotopes are administered into the blood vessels as a contract medium for imaging. By then measuring the resultant radioactive emissions, the state of blood flow and other information can be measured. However, in order to minimize adverse effects on a living body from the radioactive isotopes utilized in the measurements, isotopes with a short radioactive half-life are used. This short isotope life span means that a cyclotron is required to manufacture these isotopes in the immediate vicinity.
Near-infrared light is widely used in measuring internal living body information in view of the high transmittance provided within the body (patent document 2, JP-A No. 135825/1997 and patent document 3, JP-A No. 212115/2001) By beaming light inside a living body, the light transmitting through the body can be measured and in this way non-invasive measurement of internal body information performed. Measurement of oxygen saturation levels within the blood is possible by making use of changes in the respiratory spectrum that are one optical characteristic resulting from receiving of oxygen by hemoglobin which functions as a receptor of oxygen. This process can also be used in measuring brain activity. The action of nerve cells is a basic element of brain activity. Brain activity can be measured by measuring the changes in the respiratory spectrum of hemoglobin that result from variations in blood flow and oxygen saturation that accompany this brain nerve cell activity.
When measuring physiological reactions such as brain activity, the measured values are often absolute values rather than relative values. In making use of significant measurements of physiological reactions, an effective technique is to compare a specific active state within the body (for example moving a finger), with a different state (for example, a resting state). To create a specific active state for action within the body during measurement, a “task period” where the test subject for example, moves a finger is established. For comparison purposes, a “rest period” is also established where the body is in a resting state. These task periods and rest periods are necessary for creating specific action states for dynamic response within the body. The test subject must be able to distinguish between these two states. Therefore, if the test subject for example is in a coma, then establishing these task periods and rest periods is impossible.
However, a specific action state within the body can also be passively measured. In order to measure a sleeping state for example, data from results obtained from measuring brain waves in a sleep state by techniques such as brain wave measurement can be sorted into task periods (REM {rapid-eye-movement} sleep time, etc.) and rest periods (deep sleep time, etc.). Therefore a specific active state within the body can be measured passively even when the subject is in a coma. However, this kind of measurement requires a long time since a sufficient amount of data must be acquired for analysis.